Actually there are many reasons.
The rating describes how much people liked your game, and people liking your game is the only reason to make a game.
The games in the top 20 are very visible, more people could potentially play it and like it.
Also, you can potentially use a success in a jam as some kind of achievement when looking for a job or a partner or something.
So if you made a game that was rated 4 by 100 people and lose to a game rated 5 by 2 people, you must admit, it's kind of bad. I mean, it's the itch's fault of course, for creating a bad system.
You could also create a game, create 2 fake accounts, rate your game twice with perfect score and then do nothing and win the jam.
It's not rating, that obsesses many people, but how easily it can by exploited on this page.